1. Field of the Invention
The present invention relates to a semi-dry scrubbing process for the removal of sulfur dioxide from gases and to a filter cake used therein.
2. Description of the Related Arts
A variety of industrial and commercial plants generate pollutant-containing gas streams. Sintering and coal burning operations typically generate unacceptably high levels of sulfur dioxide (SO.sub.2). These gas streams must be cleansed or "scrubbed" before being released to the atmosphere. One of the known techniques for removing SO.sub.2 and other similar pollutants is the "semi-dry" scrubbing process.
Generally the semi-dry scrubbing process comprises contacting the gas with an aqueous solution or slurry of a reagent to neutralize the pollutant, under conditions that evaporate the liquid water carrier, for example, contacting in a spray drier. The resulting gas stream, which is cooler and more humid, will contain particulates of unreacted reagent and reaction product(s). These particulates are normally removed from the gas by a particle separator such as a baghouse to yield a scrubbed, pollutant-reduced gas.
Such a system is a compromise between the wet scrubbing process where high pollutant removal efficiencies are achieved but with high maintenance and liquid disposal costs, and the dry scrubbing process, where lower removal efficiencies are obtained. In the semi-dry process, the initial presence of water or other liquid catalyzes a more rapid pollutant transfer (capture) and neutralization (reaction) than in simply passing the gas over a solid reactant as in the dry scrubbing process. However, because the liquid is dried or evaporated, no liquid waste stream is formed as in the wet scrubbing process, thereby reducing the maintenance and disposal costs. Another disadvantage associated with wet scrubbing is the common requirement that the gas be cooled and water-saturated in order to avoid evaporation of the absorbent. The gas often must then be reheated to burn-off excess moisture in the gas before being discharged to the atmosphere so that the plume is not overly dense and offensive. As a result of these demands, the wet scrubbing process usually requires substantial equipment and energy consumption.
Suitable reagents in the semi-dry scrubbing process include lime compounds such as hydrated lime, limestone, etc., as well as alkali compounds such as sodium carbonate. The lime reagents are generally less expensive than the alkali reagents, but the alkali reagents are generally more effective. Because of these trade-offs, lime is normally used unless the required removal efficiency/reliability requires an alkali reagent or it otherwise becomes more cost effective.
While the initial aqueous contacting step generally provides the main pollutant removal effect, the subsequent particulate filter can provide supplemental pollutant removal efficiency. In a typical SO.sub.2 semi-dry scrubbing process where the gas is contacted in a spray drier with a lime slurry, about 70-80% of the SO.sub.2 is removed in the spray drier and up to about 20% of the SO.sub.2 is additionally removed in the baghouse (overall removal is about 80-90%). This additional removal is possible in the baghouse because of the contact of the gas with the accumulated filter cake present on the filter. The filter cake contains the particulates removed from the gas including the unreacted lime. Thus, the filter cake can operate as a dry scrub reactor. However, the removal efficiency is inconsistent, ranging from near zero to 40% (efficiency based on the residual amount of pollutant), and is generally independent of the ratio of the residual pollutant in the gas stream to the unreacted lime in the cake (the residual ".alpha.").
U.S. Pat. No. 4,581,210, the entire contents of which are incorporated herein by reference, teaches that introducing a sorbent such as limestone via a dry venturi into the humidified gas between the quench reactor and the baghouse increases SO.sub.2 removal efficiency in the baghouse. According to this patent, the dry venturi facilitates formation of an improved filter cake which acts as an improved secondary fixed-bed reactor in the baghouse to further purify the gas. The cake is substantially non-tacky and porous, thus allowing much greater cake buildup over the prior art systems before removal is required.
However, the gases being scrubbed in this patent generally contain HCl, as is commonly found in municipal waste incinerators. Further, the patent requires doping of the calcium based slurry used in the spray drier or quench reactor with a hygroscopic agent. Either one of these features increases the adsorption of water in the filter cake which in turn is believed to catalyze the SO.sub.2 transfer and reaction. Accordingly, such a method is not expected to be suitable for treating other types of gas streams such as those derived from sintering operations, or without the dopant.